1. Field of the Invention
The present invention relates to a method of forming a nozzle in an element for an ink jet print head, wherein, by the use of a laser, material is removed from the element and the nozzle is formed. The method includes the steps of irradiating a mask with a laser beam in such a manner that a sub-beam is passed through the mask, and removing the material by means of the sub-beam. The present invention also relates to a nozzle element with substantially identical nozzles, an ink jet print head provided with said nozzle element, and an ink jet printer provided with such a print head.
A method of this kind is known from U.S. Pat. No. 5,305,015. The element in which the nozzle is formed is used as part of a print head for an ink jet printer. A print head of this kind typically comprises a series of substantially closed ink ducts each leading from a relatively large opening into the surface of the print head. In one embodiment, these openings form a pattern of two parallel rows. A flat element is fixed against this surface of the print head and contains a number of nozzles in a pattern corresponding to the pattern of the openings. Consequently, each duct finally leads into a small accurate nozzle. Each duct is provided with drive means comprising, for example, a thermal element or a piezo-actuator, with which drive means a rapid pressure rise can be generated in the duct so that an ink drop is ejected via the corresponding nozzle. By actuating the ink ducts, image-wise, it is possible in this way to form an image, built up of a number of individual ink drops, on a receiving material.
With ink jet printers of this kind, the print quality depends very much on the characteristics of the nozzles. The shape of the nozzles, the size (cross-section) and the angle they include with the duct, particularly determine important properties of the drops. The properties include, in particular, the drop size, the direction in which the drops are ejected, and the speed that they have at the instant of ejection. In addition to providing nozzles in separate elements, for example flexible metal or plastic films that are fixed on the print head, it is also possible to form the nozzles directly in an element provided with ink ducts.
Thus an element of a flexible plastic is transported through a processing station in which a mask is irradiated with a laser beam, the mask being formed with a pattern of laser-passing elements. The laser beam originates from an excimer laser, for example of the F2, ArF, KrCl, KrF or XeCl type. A laser beam of this kind is excellent for forming nozzles because a high energy density can be obtained over a small area. The pattern of laser-passing elements in the mask results in a pattern of sub-beams which are passed through the mask. With each of these sub-beams material is removed from the tape with the formation of a nozzle. A nozzle is finished when a continuous hole of a specific shape has been formed in the element. Since the number of sub-beams originating from the laser beam is much less than the total number of nozzles to be provided, the element is moved with respect to the mask and the laser beam after a first series of nozzles has been formed, whereafter a following series of nozzles is formed. This method is known as the step-and-repeat process.
This method has a significant disadvantage. The composition of the laser beam, particularly the angle at which the radiation is propagated in the beam and the intensity of this radiation, is not exactly identical over the entire width of the beam. This means that the composition of a sub-beam passed through the mask is also not exactly known.
As a result, the characteristics of the nozzle formed by processing with this sub-beam are difficult to adjust, if not impossible. Accordingly, the spread in characteristics over the nozzles is relatively considerable. Moreover, in the known method, nozzles may be formed which deviate considerably from the required nozzle shape. For example, it is quite possible that nozzles are formed which are fairly skewed with respect to the duct or have a much larger cross-section than required. This has adverse effects on the print quality.
The object of the present invention is to provide an ink jet print head nozzle element in a simple manner, with which good print quality can be obtained. To this end, a method has been discovered wherein the laser beam is moved with respect to the mask in a direction substantially parallel to the mask so that during the removal of the material the sub-beam originates from a series of different parts of the laser beam, extending in said direction. In this method the nozzle is formed by a kind of xe2x80x9caveragexe2x80x9d of the laser beam.
For this reason, the spread in nozzle characteristics over the nozzles formed is relatively small and it is a simple matter to form nozzles which correspond well to the required nozzle shape.
In this embodiment, the laser beam is moved with respect to the mask so that the laser-passing element of the mask is always irradiated with a different part of the laser beam, so that a different part of the laser beam is also always passed as a sub-beam. In this way, deviations in the laser beam, and hence in the sub-beam with which the nozzle is formed, are averaged out over a larger number of parts of said beam. This has the advantage that the consequences of systematic faults in the laser beam can readily be eliminated. In addition, the formation of the nozzle can be controlled more easily because a change in the setting of the laser beam has less rigorous effects for the xe2x80x9caveragexe2x80x9d beam than for each of the parts in the beam.
In another embodiment, the series of different parts of the laser beam forms a contiguous row. The advantage of this method is that the mask can be continuously irradiated with the laser beam. This provides a simpler method and also has the advantage that there are no sharp transitions at the transition from one part of the laser beam to the other. Instead, the laser beam is moved over the mask in one fluid movement.
In yet another embodiment, the series extends over substantially the entire width of the beam in the said direction. Since the laser beam used is often symmetrical in respect of its properties, a very good averaging out of deviations in the laser beam takes place in this way, in which the nozzle is formed by using substantially the entire width of the laser beam. The result is a nozzle which is substantially symmetrical, and this benefits print quality.
In one preferred embodiment, the laser beam is moved with respect to the mask at a substantially constant speed. This not only simplifies the method according to the invention, but also contributes to better averaging over the beam. In this way symmetry of the nozzles is very satisfactorily guaranteed and an improvement in print quality is obtained.
In a further embodiment, the mask and the element are moved while the laser beam is stationary. In this embodiment, the laser beam can be fixed in a processing station. The mask and the element are optically fixed with respect to one another during the movement so that the sub-beam passed by the mask is always imaged on the same location of the element.
In one preferred embodiment, the sub-beam is imaged on the element by means of a lens. This embodiment has a number of advantages. Firstly, in this way a relatively coarse mask, i.e. one having a relatively large laser-passing element, can be used because any required reduction on the element can be obtained by means of the lens. In addition, in this way the radiation intensity of the laser beam at the mask can be kept relatively low, thus preventing damage to the mask. Also, the use of the lens gives greater freedom in respect of the layout of the laser beam, mask and element with respect to one another.
In a further preferred embodiment, at least two sub-beams are passed through the mask. This method has a very important advantage over the known method. In the known method, the homogeneity of the laser beam is carefully controlled and adjusted so that at least two sub-beams passed through the mask are as far as possible identical. In this way, the nozzles formed are prevented from differing from one another as far as possible, i.e. in respect of shape, size and angle. This control and adjustment require expensive measuring and control equipment, but is necessary because such differences between the nozzles result in a perceptible deterioration in print quality. In the method according to the invention, the nozzles formed have been found to be substantially identical, without having to control and adjust the homogeneity of the laser beam. This is a consequence of the fact that each nozzle is formed with substantially the same xe2x80x9caveragexe2x80x9d over the laser beam. An additional advantage of this method is that it is possible to use a very inhomogeneous and hence cheap laser beam. Particularly when the nozzles are situated in one row having the same direction as the direction in which the laser beam is moved with respect to the mask, the nozzles are found to be practically identical.
In a further embodiment, more nozzles are formed in the element than there are sub-beams originating from the laser beam. This embodiment has the advantage that a laser beam can be selected which has a small cross-section, this being relatively inexpensive. By moving the element with respect to the laser beam it is nevertheless possible to form a large number of nozzles.
In yet another embodiment, the laser beam is used in such manner that a projection of the laser beam on the mask has a longitudinal direction, the projection in the longitudinal direction being defined by substantially parallel lines. This embodiment offers the advantage that the positioning of the beam with respect to the laser-passing elements of the mask need not be as accurate: since the laser beam is substantially of equal width throughout, each nozzle will be formed with a substantially identical total laser intensity. This is to the benefit of the uniformity of the nozzles, and hence the print quality of an ink jet printer equipped with a nozzle element according to the invention.
The present invention also relates to a nozzle element for an ink jet print head, the nozzle element having substantially identical nozzles and being obtainable by a method according to the invention. An element of this kind has the advantage that the ink drops ejected from the nozzles have the same properties as far as possible. An ink jet print head provided with nozzle element of this kind has the advantage that the print properties differ as little as possible over the length of the head. With an ink jet printer provided with a print head of this kind it is possible to generate images of high quality.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.